1. Field of the Invention
The present invention relates generally to a manufacturing method and apparatus therefor to generate complex shapes on cylindrical surfaces. The invention relates more specifically to a method and apparatus for altering the geometry of the radial surface of cylindrical rollers to extremely precise specifications in a reliable and repeatable manner and in a minimum amount of time.
2. Background Art
Ceramic materials are being used to a greater degree in vehicle engines. Ceramics such as silicon nitride, exhibit physical characteristics which are extremely beneficial in the harsh environment of diesel and internal combustion engines. Engine parts made from silicon nitride are more resistant to high temperature effects because they have a lower coefficient of thermal expansion than conventional metal parts. They are also more resistant to friction and wear, have high strength and resist oxidation. Their low mass compared to parts made of ferrous metals makes them attractive in weight sensitive applications and in high speed applications. One application where silicon nitride has especially proved its advantages is in the form of rollers such as cam rollers in diesel and spark ignited engines.
To understand the benefits of silicon nitride cam rollers, one must first understand the failure mode of metal cam rollers. A metal cam roller system for heavy-duty diesel engines consists of a steel roller with a bronze pin. The metal roller runs against a steel cam. The failure begins as the bronze pin wears. The wear is accelerated because bronze is a relatively soft metal. The wear of the pin makes it more difficult for the metal roller to rotate. This then results in skidding against the cam lobe and ultimately scuffing, micro welding and galling of both the cam roller and the cam lobe. Fatigue failure will follow. Higher contact stresses of new diesel engines accelerate the above process.
A silicon nitride based system consists of a silicon nitride cam roller and a hardened steel pin. The use of a harder pin material eliminates the root cause of failure in the metal cam roller system, the bronze pin. The use of a silicon nitride cam roller eliminates the problem of wear and galling of the metal roller. Silicon nitride has a very low coefficient of friction versus steel (approximately 0.05 lubricated) and a very low mass (3.2 grams per cubic centimeter, 60% lighter than steel) which gives the silicon nitride cam roller a lower moment of inertia, making it easier to rotate against the cam and reducing the chance of skidding against the cam lobe.
Silicon nitride is very compatible with steel from a wear standpoint. The measured wear of a steel pin in a silicon nitride cam roller system is over 95 percent lower than the corresponding wear with a steel roller and bronze pin. No micro welding will occur with the silicon nitride cam roller system because of the dissimilarity of materials. Silicon nitride cam roller systems eliminate the major failure modes of metal cam rollers, and are proven to reduce cam lobe wear and increase cam lobe life, even at higher contact stresses. Additionally, silicon nitride has a better rolling contact fatigue life than bearing steels, resulting in a system with improved reliability
The reliability of silicon nitride has been demonstrated by the fact that millions of rollers are currently in engines with no reported field failures by material related problems. This excellent mechanical reliability is due to processing techniques that result in no porosity on wear surfaces. The absence of porosity eliminates the major cause of contact fatigue failure.
The cost of silicon nitride components is significantly higher than steel components. This can be addressed by minimizing the cost of raw materials through the use of sintered reaction bonded silicon nitride processing. Improvements in continuous and semi-continuous ceramic process and grinding procedures can further reduce the cost difference between silicon nitride and metal. With sufficient reduction in cost difference, silicon nitride components can be justified on the basis of the life cycle and performance improvements compared to metal components. For example, engine life costs can be reduced because problems related to fatigue in metal rollers or cam lobes during the warranty life of an engine do not arise when the ceramic alternative is used.
Silicon nitride can be a cost effective material solution that eliminates wear and galling of the cam rollers and the adjoining cam lobe, or pump rollers and the adjoining metal components. The use of silicon nitride components has eliminated warranty problems in these applications and has resulted in increased engine reliability.
Cylindrical rollers employed in engines require precise grinding to generate a crown shape on the radial surface. Otherwise, a perfectly straight radial surface, when under radial load, can result in damage from the interaction of the axial edges with the cam or other interface. Typically, a crown shape consists of a slight reduction in outer diameter with the maximum reduction at the axial edges and little or no reduction at the center of the axial length of the radial surface. The amount of diameter reduction is measured in ten-thousandths of an inch. The currently known method for grinding a precise crown shape onto a silicon nitride roller is carried out on a machine receiving a large number of ceramic rollers. The rollers are placed in contiguous relation to two almost parallel large spinning rollers having undulating surfaces and which impart a rotational force onto each of the individual rollers on the spindle. A contoured grinding surface containing diamond powder is lowered into contact with the rollers to form the crown shape on the respective radial surfaces of the rotating rollers. The undulating surfaces of the large rollers cause the workpiece rollers to tilt slightly to allow the grinding surface to create the crown shape.
This conventional process would appear to provide an efficient way of shaping multiple rollers simultaneously, but there are multiple problems as well. There are only two available directions of movement. The grinding surfaces can be moved up and down and the rollers on their mandrel can be translated along their common axis. The tilting of the rollers is relatively imprecise and also creates instability during grinding. Because of the limited freedom of movement, it is difficult to generate the desired shape on one cycle. In fact, it usually takes six or seven cycles before each roller is properly shaped. Assessment of the rollers after each cycle is required before carrying out the next cycle. Each assessment requires examination under a profiling instrument which means removing each roller from its spindle before assessing it and placing it back on a spindle before the next cycle. The cumulative effect of multiple cycles and multiple assessments causes the average grinding time per roller to be longer than desirable and to add substantially to the labor costs per roller. Moreover, the prior art crowning process is relatively imprecise thereby often resulting in roundness degradation and oversized crowns which do not meet geometric requirements. It would therefore be highly advantageous if it were possible to provide a reliable single cycle crown shaping method and apparatus with more dependable yield.